Patent foramen ovale (PFO), a residual tunnel between the right and left atria, is associated with more than 150,000 strokes per year. Traditionally, it was thought that PFOs facilitate paradoxical embolism by allowing venous clots to travel directly to the brain. However, there is a significant disconnect between this simple mechanism and clinical data, as only a small portion (10-17%) of these patients have a known tendency to form clots. Since PFOs are common in the general population (25-30% of adults), the lack of strong clinical consensus on management leaves clinicians with no clear guidance on treatment. We propose a hypothesis of a novel heart-brain signaling mechanism for PFO-related stroke: In the heart: Due to PFO-related right-to-left shunting, serotonin (5-HT) and other unfiltered substances gain direct access to the brain. In the brain: Cerebral endothelium is exposed to elevated 5-HT, and upregulates deleterious neurovascular mediators such as thrombospondin-1(TSP-1), matrix metalloproteinases (MMPs), microparticles (MPs) and more 5-HT, and downregulates neuroprotective signals such as BDNF and FGF. In the circulation: TSP-1 activates platelets, MMPs and microparticles promote endothelial dysfunction, and induce an acquired prothrombotic state. Ultimately PFO allows inappropriate signals to avoid filtration by the lungs, staying in the circulation and amplifying each other in a positive feedback loop leading to further neurovascular injury. Our pilot data seem to support our hypothesis and therefore we plan to: 1) characterize neurovascular mediators that are triggered in human cerebral endothelial cell cultures by 5-HT and examine the mechanism of 5-HT-induced neurovascular injury in vitro 2) in the heart, assess acute change in candidate neurovascular mediators in PFO stroke patients before and after PFO closure, and correlate mediators to closure efficacy. 3) explore the longitudinal circulatory proteomic profiles of PFO patients undergoing closure for correlation with clinical outcomes. Clinical endovascular closure of PFO provides a rare opportunity to explore a bedside model to manipulate PFO circulatory signaling. Our combined cell biology and translational clinical approach gives us a unique opportunity to test our novel heart-brain signaling hypothesis in PFO-related stroke.